A carbon fiber is useful as a reinforcing material for various materials because it is excellent in specific strength, specific modulus, heat resistance, and chemical resistance. The carbon fiber is used in a wide range of fields such as aerospace applications, leisure applications, and general industrial applications. Since the carbon fiber is often used in places where strength is required, the carbon fiber needs to uniformly and stably have extremely high properties, that is, requires excellent high quality.
Generally, as a method of producing a carbon fiber from a polyacrylonitrile-based fiber, the following method has been known. A fiber obtained by bundling several thousands to several tens of thousands of single fibers made of a polyacrylonitrile copolymer (hereinafter abbreviated as a precursor fiber for carbon fiber) is fed into an oxidation oven, and exposed to hot air having an oxidizing atmosphere such as air heated to 200 to 300° C., to subject the fiber to a heat treatment (oxidation treatment). Then, the obtained oxidized fiber is fed into a carbonization furnace, to subject the oxidized fiber to a heat treatment (precarbonization treatment) in an inert gas atmosphere at 300 to 1000° C. Then, the oxidized fiber is subjected to a heat treatment (carbonization treatment) in a carbonization furnace filled with an inert gas atmosphere of 1000° C. or higher. The oxidized fiber, which is an intermediate material, is widely used as a material for flame-retardant woven fabrics, taking advantage of its incombustible performance.
In the producing step of the carbon fiber, a silicone based oil agent is often used for the precursor fiber for carbon fiber to avoid the adhesion of the oxidized fibers.
Meanwhile, a hot air circulation type oxidation oven is widely used for an oxidation treatment of an industrial production scale. The hot air circulation type oxidation oven constitutes a hot air circulation system including a heat treatment chamber for subjecting a precursor fiber for carbon fiber to an oxidation treatment, and a hot air circulation path for heating and circulating hot air. Since the hot air can be repeatedly used by the hot air circulation system, the hot air circulation type oxidation oven can advantageously reduce the loss of thermal energy.
However, the hot air circulation type oxidation oven has a disadvantage that impurities such as dust are apt to stay in the hot air in the oxidation oven for a long period of time because the impurities staying in the hot air are less likely to be discharged out of the hot air circulation system. In particular, the dust generated from the silicone based oil agent applied to the precursor fiber for carbon fiber accumulates in the oxidation oven, and also adheres to the precursor fiber for carbon fiber during the oxidation treatment. The adhesion point of the dust adhering to the precursor fiber becomes the generating point of fuzz and single yarn break in the subsequent carbonization treatment, which causes significant deterioration in quality of the obtained carbon fiber.
Examples of the dust staying in the oxidation oven include aggregates of oil agent components other than the silicone oil agent, dust adhering to the precursor fiber for carbon fiber and carried from the outside of the oxidation oven, dust contained in outside air flowing into the oxidation oven, and dust made of a composite thereof in addition to dust derived from the silicone based oil agent.
If the dust stays in the oxidation oven, the quality of the carbon fibers obtained as described above is significantly deteriorated so that the carbon fiber cannot be stably produced. Furthermore, when a porous plate for rectifying a velocity is provided on a blowout surface of a hot air blowout port, the porous plate is clogged and blocked, which causes the circulation of the hot air to be delayed. If the circulation of the hot air in the heat treatment chamber is delayed, the heat removal of the precursor fiber for carbon fiber is not smoothly performed, which induces the yarn break of the precursor fiber for carbon fiber. The broken precursor fiber for carbon fiber is tangled with other precursor fiber for carbon fiber, to induce the yarn break of a precursor fiber for carbon fiber traveling in other traveling region. This leads to a fire and the like in the worst case, which impedes a stable operation of the oxidation oven.
Therefore, the conventional oxidation oven causes difficulties for a long-time continuous operation, and makes it necessary to frequently stop the operation to clean the inside of the oxidation oven, which leads to inhibition of improving the production efficiency of oxidized fiber. The maintenance cost required to clean the oxidation oven is great. In addition, while the oxidation oven becomes larger with the expansion of the amount of production in recent years, the structure in the oxidation oven becomes complicated to reduce velocity mottle in the oxidation oven for the purpose of producing a high-quality oxidized fiber and carbon fiber, which makes it difficult for operators to directly enter and completely clean all flow channels through which the hot air is circulated.
In the hot air circulation type oxidation oven, improvement in production efficiency and reduction in maintenance cost depend on how to reduce the dust in the oxidation oven. To reduce the dust, in addition to removal of the generation factors of the dust or discharge of the generated dust from the hot air circulation system and the like, efficient removal of the dust adhering to the inside of the oxidation oven can be considered.
To address this problem, for example, Japanese Patent Laid-Open Publication No. 2001-316946 proposes an oxidation oven including a water spray nozzle and a drainage port provided in the upper part of a rectifying plate of the oxidation oven to clean and remove dust adhering to the rectifying plate. That oxidation oven eliminates the need to manually spray water toward the blocked rectifying plate to which the dust adheres, to provide easy cleaning. Japanese Patent Laid-Open Publication No. 2012-201997 proposes an oxidation oven having an aggregation mechanism which takes in hot air from a hot air circulation path, aggregates impurities, and returns the hot air to the hot air circulation path again. That oxidation oven can efficiently remove the dust from the hot air, to allow the operability to be improved. Furthermore, Japanese Patent Laid-Open Publication No. 2008-231611 proposes an oxidation oven which discharges an oxidizing gas passing through a travelling region for a precursor fiber for carbon fiber at an initial stage of oxidation where dust is most generated, out of the oxidation oven without circulating the oxidizing gas. That oxidation oven can greatly reduce the dust adhering to the inside of the oxidation oven, which can provide a continuous operation for a long time. Japanese Patent Laid-Open Publication No. 8-311723 proposes an oxidation oven including an exhaust port provided in a hot air circulation path. According to that oxidation oven, hot air of a hot air circulation system is exhausted from the exhaust port out of the hot air circulation system before re-operation after the inside of the oxidation oven is cleaned, whereby the dust remaining in the oxidation oven can be reduced, which makes it possible to prevent quality of an oxidized fiber from deteriorating initially after the re-operation.
However, the oxidation oven described in JP '946 has an effect of removing a specific amount of the adhering dust, but only the water spraying causes insufficient cleaning and removing effects.
The oxidation oven described in JP '997 has an effect of reducing the dust adhering to the inside of the oxidation oven, but the treatment of the whole hot air in the aggregation device leads to upsizing of equipment, which is not preferable also from the viewpoint of the loss of thermal energy. Therefore, after all, a structure for treating only a part of the hot air becomes realistic, and the adhesion of the dust in the oxidation oven cannot be completely prevented. Since it is necessary for the operators to periodically enter the oxidation oven for cleaning, an efficient cleaning method is still required.
In the oxidation oven described in JP '611, the hot air passing through the region where the dust is most generated is discharged out of the oxidation oven, whereby the oxidation oven has an effect of reducing the dust adhering to the inside of the oxidation oven. However, the discharge of the hot air is not preferable from the viewpoint of the loss of thermal energy. Since the adhesion cannot be completely prevented, it is necessary for the operators to periodically enter the oxidation oven for cleaning so that an efficient cleaning method is still required.
The oxidation oven described in JP '723 makes it possible to prevent quality of the oxidized fiber from deteriorating initially occurring after the re-operation of the oxidation oven, but the dust in the oxidation oven cannot be completely removed. Therefore, the cleaning of the inside of the oxidation oven is still required, and an efficient cleaning method is also required.
It could therefore be helpful to provide a method of cleaning an oxidation oven, which can obtain a high-quality carbon fiber immediately after re-operating the oxidation oven, can easily clean the inside of the oxidation oven, and can reduce a period in which production is stopped, and a method of producing an oxidized fiber and a method of producing a carbon fiber including the step of cleaning the oxidation oven using the cleaning method.